One of a number of objects ranging in size from sub-km to about 1000 km, most of which lie between the orbits of Mars and Jupiter; also called ‘minor planet’. The preliminary designations consist of the year of discovery, an upper case letter to indicate the halfmonth in that year (A=Jan 1-15, B=Jan 16-31, …, Y=Dec 16-31, the letter I being omitted), and a second upper case letter in sequence. When this sequence of 25 letters (with I again being omitted) has been completed it is repeated and followed by a sequential number.Permanent designations consist of numbers and names, beginning with (1) Ceres, given to asteroids for which orbits are accurately determined. Names are generally proposed by the discoverer.See also: Apollo asteroids, Aten asteroids, Meteoroid Stream

A diffuse body of solid particles and gas, which orbits the Sun. The orbit is usually highly elliptical or even parabolic. Comets are unstable bodies with masses of the order of 10^18 g whose average lifetime is about 100 perihelion passages. Periodic comets comprise only ~4% of all known comets. Periodic comets are designated by a number, followed by ‘P/’ and its name. E.g. Halley’s comet has the designation 1P/Halley, the parent body of the Perseids, 109P/Swift-Tuttle.See also: Meteoroid Stream

Abbreviation i., in the Solar System, the angle between an orbit and the plane of the Earth’s orbit (ecliptic).

Limiting magnitude

Generally denotes the faintest star visible during an observation and evaluates the quality of the sky as well as the observing technique. The magnitude of the faintest meteor visible can be different from the stellar limiting magnitude, particularly for photographic and video observations. Visual observations assume about the same limiting magnitudes for stars and meteors.See also: Photographic observations, Video observations, Visual observations

A solid object moving in interplanetary space, of a size considerably smaller than a asteroid and considerably larger than an atom or molecule.See also: Fireball, Meteor, Meteorite, Meteoroid Stream

Meteoroid Stream

Stream of solid particles released from a parent body such as a comet or asteroid, moving on similar orbits. Various ejection directions and velocities for individual meteoroids cause the width of a stream and the gradual distribution of meteoroids over the entire average orbit.See also: Asteroid, Comet, Meteoroid

Micrometeorite

A small extraterrestrial particle that has survived entry into the Earth’s atmosphere. The actual size is not rigorously constrained but is operationally defined by the collection procedure. Micrometeorites found on the Earth’s surface are smaller than 1mm, those collected in the Stratosphere are rarely as large as 50 micro-m.See also: Meteorite

Path

The projection of the line of motion of the meteor on the celestial sphere, as seen by the observer.

Perihelion distance

abbreviation q, distance of the least heliocentric separation for a body in an eccentric orbit; q=a(1-e).See also: Apollo asteroids

Persistent train

Remaining glow due to ionization in the upper atmosphere after the passage of a meteoroid. The intensity and duration depend on the meteoroid’s atmospheric entry velocity, its size, and its composition. Bright fireballs occasionally caused trains visible for several minutes.See also: Fireball

Photographic observations

The meteors are captured on a photographic film or plate. The accuracy of the derived meteor coordinates is very high. Normal-lens photography is restricted to meteors brighter than about +1mag. Multiple-station photography allows the determination of precise meteoroid orbits.See also: Limiting magnitude, Video observations

Poynting-Robertson effect

A dissipative force due to the anisotropic loss of momentum by a particle through re-radiation of solar energy. This causes aphelion collaps such that a circular orbit is soon attained; thereafter the particle spirals slowly towards the Sun. Small particles (below 1cm) are most severely affected because the force varies as the reciprocal of its size.

Radiant

The point where the backward projection of the meteor trajectory intersects the celestial sphere. More generally, the point in the sky where meteors from a specific shower seem to come from.See also: Trajectory

Radio observations

Two main methods are used, forward scatter observations and radar observations. The first are easy to carry out, but deliver only data on the general meteor activity; showers cannot be associated. The last is carried out by professional astronomers. Meteor radiants and meteoroid orbits can be determined.

Semimajor axis

Abbreviation a, half the length of the major axis of an ellipse, a standard element used to describe an elliptical orbit.See also: Apollo asteroids, Aten asteroids

Solar longitude

Angular distance along the Earth’s orbit measured from the intersection of the ecliptic and the celestial equator where the Sun moves from south to north. It gives the position of the Earth on its orbit and, hence, is a more appropriate information on a meteor shower’s maximum than the date.See also: Meteor Shower

Telescopic observations

Monitoring meteor activity by a telescope, preferably binoculars. This technique is used to determine radiant positions of major and minor showers, to study meteors much fainter than those seen in visual observations.See also: Visual observations

Trajectory

The line of motion of the meteor relative to the Earth, considered in three dimensions.See also: Meteor Shower, Radiant

Universal Time

The local mean time of the prime meridian. It is the same as Greenwich mean time, counted from 0 hour beginning at Greenwich mean midnight.

Video observations

This technique uses a video camera coupled with an image intensifier to record meteors. The positional accuracy is almost as high as that of photographic observations and the faintest meteor magnitudes are comparable to visual or telescopic observations depending on the used lens. Meteor shower activity as well as radiant positions can be determined. Multiple-station video observations allow the determination of meteoroid orbits.See also: Limiting magnitude, Photographic observations, Visual observations